This invention generally relates to optical communication systems. More specifically, this invention relates to four-port bi-directional optical circulators.
In fiber optical networks, information transport between terminals is typically carried out by one fiber cable in one direction and another fiber cable in the reverse direction. FIG. 1A shows the information exchange between terminal A and terminal B where tightly packed channel signals xcex1, xcex2, xcex3, xcex4, . . . are transmitted from A to B through cable 1, and from B to A through cable 2. Information may also be transmitted bi-directionally, as shown in FIG. 1B. In this case, cable 1 transmits the odd-channel signals xcex1, xcex3, xcex5, . . . from A to B. Cable 2 transmits the even-channel signals xcex2, xcex4, xcex6, . . . from B to A. Bi-directional transmission is desirable because (1) for a given channel spacing, only half the fiber cables are needed; (2) for a given channel count, the channel spacing in bidirectional transmission is doubled (i.e., xcex94xcexxe2x80x2=2xcex94xcex). Consequently the cross talk or interference between channels is significantly reduced.
Therefore there is a great demand for both active and passive optical components suitable for bi-directional networks, such as bidirectional dispersion compensator, bi-directional gain equalizer, bi-directional add/drop module, etc. Bi-directional circulator is the key element for construction of the above-mentioned bidirectional components. It provides circulation of optical signals from port 1 to port 2, from port 2 to port 3, from port 3 to port 4, and from port 4 to port 1.
One prior art bi-directional circulator is described in U.S. Pat. No. 5,689,593. This circulator utilizes a polarization beam splitter to either transmit or reflect beams to different output ports according to the polarity of the beams. For a conventional broadband polarization beam splitter, the principal transmittance of the p-polarized components (polarization in the plane of incidence) is typically xe2x89xa795%, and the principal reflectance of the s-polarized beam (polarization perpendicular to the plane of incidence) is typically xe2x89xa799%. The residual reflected portion of the p-component (xe2x89xa65%) and the residual transmitted portion of the s-component (xe2x89xa61%) will end up coupling into the wrong input/output ports. The residual reflection also exists in an improved beam splitter made with two birefringent crystal prisms, as suggested in the U.S. Pat. No. 5,689,593. Therefore, this circulator suffers from cross-talk among 4 ports. In addition, the circulator is bulky since the adjacent ports are perpendicular with each other.
It is therefore the objective of this invention to provide a compact, economical bi-directional circulator with very low cross-talk, very high extinction ratio, and very high isolation.
Embodiments of the present invention are directed to a 4-port optical circulators and apparatus employing 4-port optical circulators. The circulators route optical signals from a first input/output (I/O) port to a second I/O port, from the second I/O port to a third I/O port, from the third I/O port to a fourth, and from the fourth I/O port to the first I/O port. In accordance with an embodiment of the present invention, a 4-port optical circulator may comprise first and second birefringent elements with a first polarization rotator optically coupled therebetween. A non-reciprocal polarization rotator is optically coupled to the second birefringent element, so that the second birefringent element is disposed between the first polarization rotator and the non-reciprocal polarization rotator. The circulator may further include a third birefringent element optically coupled to the non-reciprocal polarization rotator; a second polarization rotator optically coupled to the third birefringent element, the second polarization rotator having first and second half-waveplate sections and first and second optical path compensation sections; and a fourth birefringent element optically coupled to the second polarization rotator. The circulator may also include first, second, third and fourth optical input/output ports. The first and third ports may be optically coupled to the first birefringent element and the second and fourth ports may be optically coupled to the fourth birefringent element.
In accordance with an embodiment of the present invention, a four-port optical circulator may include first, second, third, and fourth input/output (I/O) ports; a first, second and third, birefringent elements; at least a first polarization rotator optically coupled between the first and second birefringent elements, and at least a first, second, and third non-reciprocal polarization rotator. The first polarization rotator has one or more reciprocal polarization rotation sections and one or more optical path compensation sections. The first non-reciprocal polarization rotator is disposed between the first polarization rotator and the second birefringent element. The second birefringent element is disposed between the non-reciprocal polarization rotator and the second non-reciprocal polarization rotator. The second non-reciprocal polarization rotator is disposed between the second and third birefringent elements. The third birefringent element is disposed between the second and third non-reciprocal polarization rotators. The birefringent elements, polarization rotators and non-reciprocal polarization rotator co-operate to route optical signals from the first port to the second port, from the second port to the third port, from the third port to the fourth port and from the fourth port to the first port.
Embodiments of the 4-port circulator provide for bidirectional optical circulation with high extinction ratio and much lower cross-talk between ports than previous designs (prior art). Furthermore, the alignment of the components is simple and the cost of construction may be reduced compared to prior circulator designs.
In accordance with another aspect of this invention, a 4-port optical circulator such as those described above may be incorporated into an apparatus with an optical signal conditioner, such as a chirped fiber grating, or wavelength converter, or optical amplifier, optically coupled to one or more of the input output ports. The chirped fiber gratings may be used to achieve bidirectional dispersion compensation. Wavelength converters may be used for wavelength switching and relocation in multi-wavelength systems.
In accordance with another aspect of this invention, a 4-port optical circulator of such as those described above may be incorporated into a bi-directional optical add/drop apparatus. Such an apparatus may include a first bidirectional circulator having first, second, third, and fourth input/output (I/O) ports. A first bandpass filter may be optically coupled to the second I/O port. A second bandpass filter optically coupled to the fourth I/O port.